System and method for powering, controlling, and communicating with multiple inductively-powered devices

ABSTRACT

Magnetic Vector Steering (MVS) and Half-Cycle Amplitude Modulation (HCAM) are novel techniques which enhance the powering and control of multiple arbitrarily oriented implant devices. Together, these techniques enable arbitrarily oriented implants to receive power and command, programming, and control information in an efficient manner that preserves battery life and transmission time while reducing overall implant device bulk. By steering the aggregate magnetic field from a near-orthogonal set of AC-energized coils, selected implants can be powered or communicated with at desired times. Communication with individual implants can also be enhanced through half-cycle amplitude modulation —a technique that allows bit rates up to twice the energizing frequency. Unlike prior art systems, power and data transfer can be realized over the same frequency channel.

This application is a division of U.S. patent application Ser. No.09/094,260, filed Jun. 9, 1998 now U.S. Pat. No. 6,047,214, disclosureof which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to techniques for enhancing the poweringof and wireless data collection from arbitrarily oriented high-bandwidthremote sensor devices such as inductively powered implant devices.

BACKGROUND ART

For the past three decades, biotelemetry has assisted many researchersand clinicians in obtaining physiological information from both patientsand animals. With the development of new electronic, communication,battery, and material technologies, the capabilities of biotelemetrysystems have expanded, bringing increased performance in the form oflonger implantation times, greater channel counts, smaller sizes, andmore robust communication.

For a majority of medical applications using biotelemetry implants, ithas sufficed to monitor only a few channels of slowly varying DC levelssuch as pressure, temperature, ion concentration, etc. or smallbandwidth signals with bandwidths typically ranging from 100 Hz to 5 kHzper channel such as EKG, EEG, EMG, etc. To date, however, biotelemetrysystems have been unable to provide the throughput necessary for certainapplications, such as cardiac mapping or high-bandwidth multichannelneural recording in which channel rates in excess of 1 Mbits/sec areoften required, thus mandating large amounts of energy to power theimplant. For long term studies, the energy requirement becomes even moreprohibitive.

Researchers are currently searching for data collection systems that canmaximize usage of developing, high-bandwidth sensor systems. Such sensorsystems include flexible plastic substrate-based biosensor arrays forbiopotential recording, and silicon-based micro-electrode arrays forneural recording.

Also of interest is the ability to collect physiological informationfrom a variety of locations within a subject. This requires a network ofsensors placed throughout a region under study. In cardiac mapping, forexample, several electropotential arrays may be required at differentischemic or infarcted areas of a heart in order to simultaneouslymonitor electrical activity during a cardiac event. A desirableimplementation for this network has each sensor as a separate telemeter,thereby eliminating the need to interconnect wires among the sensors.The elimination of these wires significantly reduces overall implantbulk and complexity while facilitating implantation.

A fundamental difficulty in developing a high-bandwidth biotelemetrysystem pertains to implant power consumption. In contrast tolow-bandwidth systems, a high-bandwidth system must transmit many morepulses in a given time-period, thus depleting the power source muchfaster. In addition, the electronics required to sample, process, andencode the sensor data will also draw more energy as the aggregatebandwidth increases. The increased power demands require the use oflarger implant batteries or alternative power sources. A popular widelyknown alternative to relying exclusively on batteries to power animplant is Inductive Power Transfer (IPT).

Inductive Power Transfer uses an AC-energized coil to create a magneticfield that couples with a receiving coil of an inductively powereddevice. The induced signal appearing at the output of the inductivelypowered device coil is then rectified and filtered to create arelatively constant DC power source. The “loosely-coupled transformer”link provides a means of eliminating and/or recharging inductivelycoupled biomedical implant batteries or capacitors. This technique hasbeen used not only for biotelemetry devices, but also for artificialhearts, ventricular assist devices, various forms of neural stimulators,and battery recharging.

What is needed is a system which can accurately target arbitrarilyoriented inductively powered devices in order to provide power to, andcommunicate at high data rates with, the arbitrarily orientedinductively powered devices.

DISCLOSURE OF THE INVENTION

The present invention pertains to a system capable of high-bandwidthcommunication and omnidirectional power transfer to a network ofarbitrarily positioned inductively powered devices. Magnetic VectorSteering (MVS) and Half-Cycle Amplitude Modulation (HCAM) are two noveltechniques which enhance the powering and control of multipleinductively powered devices. Together, these techniques enablearbitrarily oriented inductively powered devices to receive power andcommand/programming/control information in an efficient manner thatpreserves battery life and transmission time. By directing the aggregatemagnetic field, using magnetic vector steering, from a near-orthogonalset of AC-energizing coils, selected inductively powered devices can bepowered and/or communicated with at desired times. Communication withindividual inductively powered devices can also be enhanced throughhalf-cycle amplitude modulation—a technique that allows bit ratetransfers up to twice the energizing frequency. The present inventioncombines power and data transmission circuitry more effectively than theprior art while also significantly reducing the hardware required of aninductively powered device such as a biomedical implant thereby reducingoverall implant bulk.

It is an object of the present invention to provide a system forremotely powering one or more inductively powered devices such that theoverall bulk of such devices is significantly reduced.

It is a further object of the present invention to provide a datacommunication system which can modulate a power carrier, communicatingwith one or more inductively powered devices, with a serial data streamat upto twice the cycle rate of the power carrier.

It is a still further object of the invention to allow the use of asingle frequency channel for both power and data transfer to arbitrarilyoriented inductively powered devices.

Some of the objects of the invention having been stated, other objectswill become apparent as the description proceeds, when taken inconnection with the accompanying drawings described as follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cylindrical “long solenoid” magnetic coil as oneexample of a magnetic coil which provides control of a magnetic vectoralong the z-axis;

FIG. 1B illustrates a cylindrical “saddle coil pair” as one example of amagnetic coil which provides control of a magnetic vector along thex-axis;

FIG. 1C illustrates a cylindrical “saddle coil pair” as one example of amagnetic coil which provides control of a magnetic vector along they-axis;

FIG. 2 illustrates an energizing coil assembly integrated into a harnessas worn by, for instance, a dog;

FIG. 3 illustrates one embodiment of an energizing coil assembly;

FIG. 4 illustrates a region, such as, for instance, a heart havingmultiple implant sensor devices about its periphery;

FIG. 5 illustrates one type of inductively powered biomedical implantdevice geometry suitable for use with the present invention;

FIG. 6 illustrates a second type of inductively powered biomedicalimplant device geometry suitable for use with the present invention;

FIG. 7 illustrates a simplified block diagram of the magnetic vectorsteering (MVS) system according to the present invention;

FIG. 8 illustrates a more detailed block diagram of the magnetic vectorsteering (MVS) system according to the present invention;

FIG. 9 illustrates a simplified schematic of an energizing coil havingassociated Half-Cycle Amplitude Modulation (HCAM) circuitry according tothe present invention;

FIG. 10 illustrates a more detailed circuit schematic of the Half-CycleAmplitude Modulation (HCAM) system according to the present invention;and

FIG. 11 illustrates a harmonic spectra graph of coil current for varyingcoil Qs according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is described more fully hereinafter with referenceto the aforementioned drawings, in which preferred embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein.

Although inductive links such as Inductive Power Transfer (IPT) havebeen used extensively for both powering of and communication withinductively powered devices, there are several limitations with respectto their use. When multiple and/or arbitrarily positioned inductivelypowered devices are energized by an external coil(s), a burdensomediversity scheme is required to ensure that a sufficient degree ofmagnetic coupling exists between the external “energizing” coil(s) andthe inductively powered device coil(s).

Two approaches have typically been used. One common approach is to usemultiple energizing coils excited at different frequencies such that thecollection of energizing coils possess near orthogonal magnetic vectorcomponents. Another approach is to use a single energizing coil andmultiple, orthogonally-oriented, receiving coils.

The chief drawback to the former technique is that the multipleenergizing coils are unsynchronized in operation (i.e. driven atslightly different frequencies), resulting in a magnetic field spanningin all directions, thus wasting source energy. The chief drawback to thelatter technique is that an increase in implant (i.e., inductivelypowered device) volume, and complexity must be afforded due to havingmultiple receiving coils for each device.

The Magnetic Vector Steering (MVS) scheme of the present invention usesan energizing coil assembly that supplies power to specific inductivelypowered devices (stimulators, telemeters, etc.) through thesuperposition of magnetic fields from separate energizing coils. Unlikeprevious systems, however, power transfer is not restricted by theorientation of the inductively powered devices, with only a singlepower-receiving coil. Moreover, the energizing coils are synchronizedand therefore operable over the same frequency channel eliminating theproblem of having the energizing coil magnetic field propagateinefficiently in all directions. An assembly of external coils isarranged to strategically maneuver a net magnetic field toward aspecific inductively powered device within a set of such devices. Thistechnique conserves source energy, since the magnetic vector is keptfrom wandering in directions where energy transfer to the inductivelypowered devices is minimal.

In one exemplary embodiment, the external coil assembly was chosen toconform to a cylinder integrated into a harness to be worn by an animal.The coils that comprise the assembly for this embodiment include a “longsolenoid” and two pairs of “saddle coils”. Each of these coils has beenshown to exhibit magnetic fields highly uniform both in magnitude anddirection throughout the majority of their respective interiors. Thelong solenoid coil illustrated in FIG. 1A provides control of themagnetic vector along the z-axis of the cylinder, while the saddle coilsillustrated in FIGS. 1B and 1C provide control in the x-axis and y-axisof the cylinder, respectively.

FIG. 2 illustrates a harness 20, as worn by a dog, which includes thecoils of FIGS. 1A-C. The harness 20 is powered, in this instance, by abattery pack source 22. Other coil types and harnesses, however, may beused in conjunction with the present invention. That is, the combinationof two saddle coils and a long solenoid coil described above need not bethe only energizing coil implementation. Any coil assembly that candirect a magnetic vector along the x-axis, y-axis, and z-axis willsuffice.

FIG. 3 illustrates a coil assembly which can be integrated with aharness system like the one illustrated in FIG. 2. The coil assembly isa single unit which includes a plurality of separate coils such that thecollection of coils is capable of radiating a magnetic field in the x-,y-, and z-axes. By varying the energy supplied to each coil, themagnitude and direction of the resulting magnetic field vector can becontrolled. Using this technique together with knowledge of theorientation of a set of inductively powered devices, the presentinvention is capable of targeting a specific device to provide power toand moving to each device in the set in a prescribed or adaptivepattern.

FIG. 4 illustrates a sphere 40 which could represent, for instance, theheart of an animal. About the sphere 40 are a plurality of arbitrarilyoriented inductively powered coil devices 42. These devices 42 can takethe form of biomedical implants which monitor various characteristics ofthe animal's heart. Upon sufficient coupling with an outside source suchas the coil assembly of FIG. 3, these implants will be able to powerthemselves and even transmit data back to an external receiving device(not shown) for diagnostic processing. The system can be programmed withthe, locations of each implant so the coil assembly can be energized atspecific levels such that each implant device is coupled with theenergizing coil assembly. The program can be “adaptive”, monitoring theenergy status of inductively powered devices and scheduling time-slotsor dwell times for energy transfer.

FIGS. 5 and 6 illustrate two examples of inductively powered devicessuitable for biomedical implantation applications. Referring now to FIG.5, the implantable device 50 appears key-like in shape and has areference electrode 52 positioned near the tip of the device 50. Anetwork of sensor sites 54 span the tip and “key” area and areelectrically connected to an integrated circuit chip 56. Integratedcircuit chip 56 in turn is connected to a coil/antenna 58 which servesto couple with an external energizing coil (not shown) in order toreceive power and also to send data out to a receiving device (notshown). Referring now to FIG. 6, implantable device 60 is circular inshape having its coil 61 wrapped about the outer periphery of thecircular area. Telemetry and power conditioning circuitry 62 connectsvia metal interconnection traces 63, to coil 61 and signal processingelectronics 65. Lastly, a power supply filtering capacitor or a smallbattery 66 is shown coupled to the coil 61. The power supply filteringcapacitor or a small battery 66 can charge itself and power the implantfor a period of time should the external energizing coil (not shown) betemporarily de-activated or de-coupled.

A simplified block diagram of the magnetic vector steering system isillustrated in FIG. 7. A reference power source 70 is used to provide astable clock source that generates a power carrier frequency signal. Thesignal from the reference power source 70 is fed to both amplitudecontrol 72 and phase control 74 blocks. The output of the amplitudecontrol block 72 is passed to a power amplifier 76 in order to providesufficient drive capabilities for an energizing coil 78. The energizingcoil 78 then sends feedback to both the amplitude 72 and phase 74control blocks in order to stabilize the system. A similar configurationis used for each of the coils that comprise the coil assembly.

For the present magnetic vector steering (MVS) system, amplitude controlranges over a factor of ten (10), while phase ranges from nearly −180°to 180°. Phase shift results while changing the pulse width of thedriving waveforms.

Referring now to FIG. 8, popular compensation techniques such as thosedescribed below are either undesirable or impractical for the presentinvention. Altering the driver reference frequency will not accommodatevariations in resonant frequencies of all coil circuits, both externaland implanted. Tuning diodes (varactors) are undesirable, since they arehighly non-linear over large voltage swings and their biasing isimpractical. Likewise, transductor-based compensation is alsoundesirable on account of similar non-linearity problems as well asincreased weight, size, and power consumption (a strong DC current isrequired to alter a transductors inductance).

The present invention, however, stabilizes coil currents in bothmagnitude and phase through the use of two feedback loops. One feedbackpath acts to compensate for magnitude variations through automatic gaincontrol. The other feedback path nulls-out phase errors by using adelay-locked loop (DLL).

Automatic gain control 82 is achieved by detecting the amplitude 84 of aparticular energizing coil current and comparing 86 it to a pre-selectedvalue, V_(m). The resulting error term modulates a pulse width of apulse-width modulator 88 (PWM), thereby changing the amplitude of thefirst harmonic at its output. The pulse-width modulator drives a Class-Dpower amplifier 90, which in turn drives a resonant coil network 92. Themagnitude of the output resonant circuit is related to the amount offirst harmonic in the PWM power signal. Hence, changes in the amplitudeerror signal spur counter changes in the amplitude of the resonant coilnetwork 92.

With regard to the phase compensation circuit 100, a delayed locked loop(DLL) corrects for phase shifts by comparing the phase of the referencefrequency with that of the coil current. Phase error is used to drive avoltage-controlled delay line (VCDL) 104, thus varying the phase of thePWM input signal, the power driver circuit, and hence, the resonant coilnetwork phase. In this way, changes in the phase error signal bringabout counter changes in the resonant coil network phase.

As shown in FIG. 8, this dual-feedback system requires that bothfeedback mechanisms operate in conjunction with one another withoutmaking the system unstable. This is because the gain compensation block82 uses the output signal from the phase correction block 100 as itsinput reference frequency.

The delay locked-loop portion of the phase control block 100 is afamiliar block seen in other delay locked-loop applications. Itcomprises a phase detector 102, a low-pass filter 106, and avoltage-controlled delay line (VCDL) 104. A summation node 108 has alsobeen added in order to allow for user adjustment of the phase.

The phase detector 102 most suitable is an XOR-type detector, augmentedwith lead/lag detection. This type of detector can indicate phasedifferences from −pi to +pi radians. Because the output of this detectoris comprised of digital logic pulses of varying width, as well as alead/lag bit, it must be used in conjunction with a low-pass filter 106that removes the AC component of the XOR output signal. The lead/lag bitcontrols the polarity of the gain. Under real circumstances, however, asmall ripple penetrates the low-pass filter 106, thus contributing tounmodeled error in the system.

The output of the phase detector 102 and low-pass filter 106 is fed to asummation node 108, where a phase offset component can be added. Such aninput is desirable for setting the bias level of the voltage controldelay line (VCDL) 104 that follows, so that VCDL operation can occur atits center or most linear region of operation. The voltage-controlleddelay line (VCDL) 104 delays the reference signal by a phase that isproportional to an input control voltage.

The Magnetic Vector Steering (MVS) system described above is capable ofpowering one or more inductively powered implant devices such as, forinstance, certain biomedical implants using a fixed or adaptivescheduling algorithm. If the total energy requirements of a set ofimplant devices can be met by an energizing coil assembly, then astandard round-robin scheduling method can be employed to inductivelypower each implant device with the energizing coil assembly dwell timeon each implant device being proportional to its energy requirements.Another standard method would be to create fixed energizing coilassembly dwell time segments, and to allocate a number of these specificsegments to each implant device.

Adaptive scheduling may also be used to power the implant devices incases when the energizing coil assembly can not meet the power needs ofall of the implant devices. If each implant device can communicate itsstored energy status to the energizing coil assembly, then the implantdevices with the highest priority can be scheduled for dwell time on anas needed basis. Such an adaptive scheduling system would be highlyeffective in applications for which the energy requirements forindividual implant devices are time-varying.

Another limitation of prior art inductively powered link systems is thelimited bandwidth of the energizing coils. Ordinarily, medium-Q tohigh-Q coils are resonated to maximize coupling efficiency and removeundesired harmonics. The Q of a coil represents the resonant peak ofcircuit response. Narrowband coils, however, restrict the communicationrate of the link, due to the increase in response time with coil Q. Forexample, it can be easily shown that for the magnitude of an energizingcoil current to settle within 5% of its final value (indicative of acomplete bit transition in AM), it would take a number of cycles of thepower carrier roughly equal to the Q of the coil. Hence for a nominalenergizing coil Q of 50, fifty power carrier cycles would be required toregister a bit transition. This response time can greatly reduce systemthroughput if inductively powered devices are required to transmitinformation upon command of the external energizing coil assembly.

The present invention, however, allows bit transitions to occur up toevery half-cycle of the power carrier using a technique known asHalf-Cycle Amplitude Modulation (HCAM). This greatly decreases the timerequired to send command information to, or otherwise communicate with,inductively powered devices.

The principle behind half-cycle amplitude modulation is that current canbe made to circulate within all or a fractional number of turns of anenergizing coil or coils, thus amplitude modulating the magnetic field.However, when a portion of the coil is instantaneously removed orswitched out from the RLC circuit, the circuit dynamics are changed. Theremoval of a coil section decreases the inductance of the circuit, whichin turn shifts the band-pass filter spectra toward infinity. As aresult, the driving frequency no longer coincides with the RLC resonantfrequency, and changes in both current amplitude and phase manifest.These changes must also “settle” according to the band-passcharacteristics of the RLC network, thus requiring a settling time.

If, however, an inductor with equivalent impedance (both real andimaginary) to the removed section is substituted or switched in for theremoved section, the circuit dynamics will remain the same. The key tothe substitution (or switching) is to perform it at a strategic instantwhen it can be made undetectably. In this way, there are no step changesin network dynamics experienced by the RLC network; hence, there is notime lost to transition recovery.

The instant to switch the inductors would be when the inductor currentis zero. This occurs twice during a given cycle of the power carrier;hence, it becomes possible to transmit two bits of data per cycle. Anominal inductive link with a 1 MHZ power carrier frequency can supportan in-link data transfer rate of 2 Mbps which has not been achieved inprior art systems.

To recover the power carrier signal, synchronous AM can be used. Bandwidth limitations of the signal-receiving coil can be eliminated if thecoil is not resonated. The voltage gain experienced through resonationis irrelevant in this circumstance due to the inherent strength of thetransmitted signal.

A Class-D amplifier is best suited for this type of application, sinceit achieves much higher power conversion efficiency than the mostefficient linear-type amplifiers (Class-C). One drawback, however, of aClass-D amplifier is that harmonic generation is prevalent. Fortunately,the band-pass filtering performed by the resonant energizer-outputcircuit removes most harmonic content, particularly at much higherfrequencies, where it is potentially detrimental due to conflicts withtelemeter transmissions.

Half-cycle amplitude modulation maintains the linear system behavior ofthe energizing resonant circuits (comprised of an energizing coil andassociated resonating capacitor), while instantaneously modulating theamplitude of the emanating magnetic field. This is done by switching-outa section of the energizing coil, while substituting in its place anequivalent inductor that does not contribute magnetic field to the link.The substitution must be accomplished at zero crossings of the coilcurrent when the inductor stored energy is zero.

A simplified circuit schematic of the HCAM system is illustrated in FIG.9. A square wave voltage signal 110 is fed through gate drive circuitry112 and a pair of power MOSFETs 114. The output of the power MOSFETs issensed and passed through modulation control circuitry 116 beforedriving a resonant RLC circuit where inductor 118 represents theenergizing coil. A secondary coil 120 is connected to the energizingcoil at a tap point such that the inductance of the secondary coil isequivalent to the tapped out portion of the energizing coil. A pair ofbilateral switches 122 are included, one connected to the energizingcoil 118 and the other connected to the secondary coil 120. The switches122 are also connected to the modulation control circuitry 116.

A more detailed circuit schematic used to implement the HCAM concept isshown in FIG. 10. It comprises a switch controller (U1), MOSFET drivercircuitry (U2-U4), and power MOSFETs (M1-M4) for coil driving andcurrent steering. U4 is an H-bridge driver chip designed to drive 4n-channel power MOSFETs. Two H-bridge channels are used to drive ahalf-bridge (M6 and M7) which in-turn generates the square-wave voltagewaveform for the energizing circuit. A simple TTL-level clock signal ispresented to pin 6 of U1 for driving the output at the desiredfrequency. Each of the implemented bilateral switches comprise ann-channel and p-channel MOSFET and four diodes. The complementaryMOSFETs are needed so that current can pass in either direction throughthe switch. With respect to the left-most switch (indicated by thesignal “HighReturn”), diodes D5 and D6 act to protect MOSFETs undersevere reverse bias conditions. Diodes D1 and D3 block current frompassing through the reverse direction, in the case when a MOSFET isturned off. The “LowReturn” switch operates in an identical manner.

N-channel MOSFETs in the bilateral switches are driven by the tworemaining channels of U1. In order to drive the p-channel devices, levelshifting must be achieved, as they are turned-on by negative voltages.The negative driving voltages are generated through a MOSFET driver chip(M3) that is referenced to −Vss, rather than ground. Level-shiftedcontrol signals are generated by the multiplexers of U2 that convert theTTL-level inputs down to −Vss (low) and 0 volts (high).

The driving signals are generated by the GAL U1. When presented with thephase polarity of the energizing current waveform, a data bit, and aclocking signal (derived from the energizing-current waveform), theprogrammed finite-state matching algorithm causes the bilateral switchMOSFETs accordingly. To avoid the need for exact switch timing, theMOSFETs are switched during the half-cycle before they will be used. Forexample, if a high-to-low transition is to be achieved in the radiatedmagnetic field, then sometime during the high, positive phase when M1 isactive, M4 will be turned on. Thus current is steered automatically asthe data transitions from high to low. Once current is flowingexclusively through M4, M1 can be turned off (unless it is needed for a“high” signal immediately following the current phase).

For the ideal case of a sinusoid alternating between two magnitudes (A₁and A₂) every half cycle, one can obtain the frequency spectra accordingto the following:${{{S\left( {j\quad \omega} \right)} = {\frac{A_{2} - A_{1}}{\pi} + {j\quad \frac{A_{1} + A_{2}}{4\quad}}}}\left\lbrack {{\delta \quad \left( {\omega + \omega_{0}} \right)} - {\delta \quad \left( {\omega \quad - \omega_{0}} \right)}} \right\rbrack} + {\frac{A_{1} - A_{2}}{\pi}{\sum\limits_{{n = 2},4,{6\ldots}}\frac{{\delta \quad \left( {\omega + \omega_{0}} \right)} - {\delta \quad \left( {\omega \quad - \omega_{0}} \right)}}{n^{2} - 1}}}$

The equation above is comprised of three parts: a DC term, a firstharmonic term, and higher-order harmonics. One should note the fall-offin harmonic amplitude with respect to frequency is related to 1/(n²−1).Therefore, harmonic power falls-off even more rapidly as the square ofthis quantity. Low-harmonic content of the power carrier signal attelemeter transmission frequencies is important since implant devicetransmissions could potentially go undetected due to the overpoweringpresence of power carrier harmonics. The higher the Q the more harmonicrejection of the square wave will occur as illustrated in the frequencyspectra graphs of FIG. 11.

The foregoing descriptions of MVS and HCAM are complementary of oneanother and can be integrated into a single system such as those forbiomedical applications. In cardiac mapping, for instance, MagneticVector Steering (MVS) can be used to provide power to and communicatewith biomedical implants placed in regions of interest about the heartof a patient such as ischemic or infarcted areas. Half-Cycle AmplitudeModulation (HCAM) can then be used to communicate at higher bandwidthdata rates with the biomedical implants powered under MVS.

The preferred embodiment of the present invention is intended forbiomedical applications, such as, but not limited to, cardiac mapping.It is, however, suitable for any inductively powered applicationrequiring remote powering of a device and/or data exchange with a remotedevice. As such, this applies to virtually any situation in whichbattery-less devices are to operate within a restricted environment.

In the claims, means-plus-function clauses, to the extent they arerecited, are intended to cover the structures described herein asperforming the recited function and not only structural equivalents butalso equivalent structures. Therefore, it is to be understood that theforegoing is illustrative of the present invention and is not to beconstrued as limited to the specific embodiments disclosed, and thatmodifications to the disclosed embodiments, as well as otherembodiments, are intended to be included within the scope of theappended claims. The invention is defined by the following claims, withequivalents of the claims to be included therein.

What is claimed is:
 1. An amplitude modulation method of transmittingdata at up to two bits per cycle of a power carrier signal having twozero crossing points per cycle for use with inductively powered systems,said method comprising the steps of: (a) detecting when the currentthrough an energizing coil is zero; (b) switching in or out a section ofsaid energizing coil in order to modulate the magnetic field of saidenergizing coil at instants when the energizing coil current is zero;and (c) replacing said switched energizing coil section with anequivalent inductance.
 2. A amplitude modulation method of transmittingdata at up to two bits per cycle of a power carrier signal having twozero crossing points per cycle for use with inductively powered systemsemploying advanced switching to compensate for inherent timing delays toestablish a desired circuit path at the instance of the zero crossingstate, said method comprising the steps of: (a) switching in or out, inthe opposite phase, a section of an energizing coil in order to modulatethe magnetic field of said energizing coil prior to a zero crossingpoint; and (b) replacing said switched energizing coil section with anequivalent inductance.